Screening unit for half-tone color reproduction



' June" 30, 1970 s. w-lJoHN o ET AL 3,517,596

SCREENING UNIT FOR HALF-TONE COLOR REPRODUCTICN Filed Dec. 27, 1967 4Sheets-Sheet 1L .Mmv/A/ 4. 1.55.00!

s. w. JOHNSON ET AL June 30, 1970 SCREENING UNIT FOR HALF-TONE COLORREPRODUCTION Filed Dec. 27, 1967 4 sheets sheet 2 Mam/w 4, 1 2mm 5 W1 mmm w \w M i w g O June 30, 1970 s. W.VJOHNSON ET AL 3,517,596

SCREENING UNIT FOR HALF-TONE COLOR REPRODUCTION Filed Dec. 27 967 4Sheets-Sheet s v I IV VEN TOR: 5/60/90 JZ/M/mA/ i MflEV/A/ f]. 1 500.ATTOIUEY June 30, 1970 I s. w. JOHNSON E AL 3,517,595

SCREENING UNIT FOR HALF-TONE COLOR REPRODUCTION Filed Dec. 27, 1967 4Sheets-Sheet v mvnv Toni Swazi/0. Jam/$0M f Mew/A. Alina M/JW ATTORNEY3,517,596 SCREENING UNIT FOR HALF-TONE COLOR REPRODUCTION Sigurd W.Johnson, Trenton, N.J., and Marvin A. Leedom,

Warminster, Pa., assignors to RCA Corporation, a corporation of DelawareFiled Dec. 27, 1967, Ser. No. 693,777 Int. Cl. G03b 33/00 US. Cl. 9512.212 Claims ABSTRACT OF THE DISCLOSURE A screening unit for use in ahalf-tone reproduction process comprises a carriage member and a rotarymember supported for rotation about its axis within a through opening inthe carriage member. Roller means support the carriage member over arecording element so that a half-tone screen, adjacent one end of therotary member, is a predetermined distance from the recording element atan exposure station. Pawl and ratchet wheel means rotate the rotarymember intermittently when the screening unit is moved along apredetermined path. The screening unit is adapted to use in combinationwith a reciprocating processing head to which the screening unit can becoupled selectively by magnetic means.

BACKGROUND OF INVENTION This invention relates generally to screeningunits used in the image reproduction arts, and more particularly to animproved screening unit for use in a half-tone process for reproducing acolored image on a recording element. The improved screening unit isparticularly useful in combination with apparatus for making amulti-colored electrophotographic print from a colored, continuous tonetransparency, such as a 35 mm. Kodachrome colored transparency or thelike.

It has been proposed to use a photographic half-tone screen, that is, ameshwork of lines at right angles, ruled on glass, in a half-toneprinting process to translate the varying (continuous) tones of anoriginal image into dots of uniform tone but varying size on areproduction of the original image. The half-tone process is usedpreferably to reproduce images on recording elements that haverelatively poor continuous tone characteristics. In a multi-color,half-tone, printing process, either electrostatic or photographic, aplurality of half-tone colored images, each of a different color, areprinted separately, in a predetermined order and in superimposition, tobuild up a composite multi-colored image corresponding to an UnitedStates Patent O original multi-colored subject. Since each of theseparate SUMMARY OF INVENTION The improved screening unit is adapted foruse with a recording element of the type that is exposed to a lightimage in a reproduction process. One embodiment of the improvedscreening unit comprises a carriage member and a rotary member supportedby the carriage member and disposed for rotation about its axis. Ahalf-tone screen is disposed adjacent one of the ends of the rotarymember, and means are provided to support the carriage member over therecording element with the half-tone screen spaced a predetermineddistance from the recording element.

In another embodiment of the invention the improved screening unit iscombined with a processing head to which it can be coupled selectivelyfor movement therewith over the recording element and for rotating thescreen automatically.

The improved screening unit is particularly, although not exclusively,adapted for use with reproduction processing apparatus wherein thescreening unit can be moved into, and out of, an exposure stationwithout touching the'recording element, and wherein the half-tone screenmay be rotated automatically through predetermined angles in preparationfor particular exposures.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of animproved screening unit in combination with electrophotographicprocessing apparatus, the latter being illustrated with some partsbroken away to expose internal parts;

FIG. 2 is an enlarged, side elevational view of the improved screeningunit of FIG. -1 in combination with a portion of the apparatus thereof;

FIG. 3 is an enlarged perspective view of the processing head shown inFIGS. 1 and 2;

FIG. 4 is a perspective view, viewed from the bottom, of the processinghead shown in FIG. 3;

FIG. 5 is an enlarged perspective view of the improved screening unitand the processing head shown in FIGS. 3 and 4;

FIG. 6 is a fragmentary cross-sectional view of the screening unit andthe base on which it is mounted, taken along the line 6-6 in FIG. 5;

FIG. 7 is an enlarged, fragmentary view of the ruled portion of thehalf-tone screen of the improved screening unit, viewed in the directionof the arrow A in FIG. 6; and

FIG. 8 is a schematic diagram of a portion of an electrical controlcircuit including means for moving the processing head and forenergizing the coupling means to couple the improved screening unittothe processing head.

DESCRIPTION OF THE PREFERRED EMBODIMENTS General An improved screeningunit is herein described in combination with electrophotographicapparatus of the type with which it is preferably adapted to cooperate,that is, with apparatus that has a reciprocating processing head.

Referring now to the drawings, particularly to FIGS. 1 and 2, there isshown apparatus 9 for making a halftone, multi-colored,electrophotographic print from a colored photographic transparency withthe aid of an improved screening unit 10. The apparatus 9 comprises aplaten or base 12 for supporting an electrophotographic recordingelement 14 of the type used in the electrophotographic arts. Afundamental explanation of an electrophotographic method and a recordingelement of the type used by the apparatus 9 is described in an article,Electrofax, Direct Electrophotographic Printing on Paper, by C. J. Youngand H. C'. Greig, in RCAReview, December 1954, Vol. XV, No. 4.

The recording element 14 includes a photoconductive surface 18 and isdisposed in a fixed relationship with a light image projector 16, suchas a photographic enlarger, so that a light image can be projected ontothe recording elements photoconductive surface 18. The projectorincludes a filter wheel 19 for disposing a desired one of a plurality ofdifferent colored filters in the path of the projected light image. Thebase 12 also supports the screening unit 10 and a processing head 20.The processing head 20, shown in detail in FIGS. 3 and 4, is adapted tobe reciprocated over, and slightly spaced from, the recording element 14to process it, and, in addition, to move and to operate the screeningunit 10 by selective coupling thereto, in a manner to be describedhereinafter in detail.

The screening unit Referring now particularly to FIGS. 5 and 6, theimproved screening unit is shown on the base 12, decoupled from theprocessing head 20, and in the park (out of use) position. The screeningunit 10 comprises a carriage member 22 and a rotary member 24,preferably a tubular member, disposed for rotation about a centralvertical axis within a through opening 26 in the carriage member 22. Thecarriage member 22 comprises a relatively thick plate 28 formed with aninwardly extending flange 30 adjacent the lower surface 32 of the plate28, narrowing and defining the lower portion of the through opening 26and providing an annular shoulder on which the rotary member 24 isdisposed.

Means are provided to maintain the carriage member 22 a predetermineddistance over the base 12 and the recording element 14 and to allow thecarriage member 22 to roll along a predetermined straight path on thebase 12. To this end, four roller bearings are fixed to the plate 28within four recessed corners 34-37 thereof. Thus, wheels or rollerbearings 38 and 40 are fixed to vertical side walls in the recessedcorners 34 and 35, respectively, as illustrated in FIG. 5. Similarroller bearings of which only roller bearing 41 is shown in FIG. 6, arefixed to the vertical side walls in the recessed corners 36 and 37.

Guide means are provided to maintain the carriage member 22 along apredetermined path over the base 12. To this end, strips 42 and 44 of amaterial having a relatively low coeflicient of friction, such asTeflon, for example, are fixed to opposite vertical sides 46 and 48,respectively, of the plate 28 and extend therebelow. The Teflon strips42 and 44 substantially touch vertical, parallel walls 50 and 52,respectively, of the base 12 and serve to guide the carriage member 22in its movement along the base 12, that is, along a path parallel to theparallel walls 50 and 52 of the base 12.

A rod 54 is slidably mounted within a fixed housing member 56 (FIG. 5)on the plate 28 of the carriage member 22, for slidable, reciprocalmotion substantially parallel to the direction of travel of the carriagemember 22. Therod 54 is spring-biased by a pin 58, extending through therod 54 near one end 60 thereof, and a pair of symmetrically disposedsprings, of which only one spring 62 is shown in FIG. 5. One end of thespring 62 is anchored to the housing member 5 6 and the other end isanchored to one end of the pin 58. Thus, the rod 54 is spring-biased ina direction away from the processing head 20, that is, to the left,looking at FIG. 5.

A gravity-actuated pawl 64 is loosely coupled to the end 60 of the rod54 by a pivot pin 65 extending through the pawl 64 and the rod 54. Thepawl 64 is adapted to engage a plurality of annularly arrayed ratchetteeth 66 formed in the upper end 68 of the rotary member 24. A screw 70is threaded in the opposite end 72 of the rod 54 for adjusting purposes,as will be hereinafter explained. The head of the screw 70 is adapted toengage a fixed member, such as a bracket member 74, fixed to theapparatus 9, for exerting a force on the rod 54, against the springtension of the spring 62, when the screening unit 10 is moved to thepark position, that is, to the extreme left position, as shown in FIG.5.

A pair of spaced-apart brackets 76 and 78 of magnetic material aredisposed'on the upper surface 80 of the plate28 of the carriage member22 so that vertical planar surfaces 82 and 84 of the brackets 76 and 78,respectively, are in substantially the same plane. The brackets 76 and78 provide means to couple the screening unit 10 to the processing head20, in a manner to be hereinafter explained.

The rotary member 24 is formed with an outwardlyextending flange 86(FIG. 6) adjacent the upper end 68 thereof, and the ratchet teeth 66 areformed on the upper surface of the flange 86. Friction reducing means,such as small rounded studs 88 of Teflon, are disposed between theflanges 86 and 30 of the rotary member 24 and the carriage member 22,respectively, and also between the outer cylindrical wall 89 of therotary member 24 and the flange 30 of the carriage member 22. Thisarrangement reduces friction between the rotary member 24 and thecarriage member 22 and also disposes the rotary member 24 accuratelywithin the carriage member 22 so that the rotary member 24 may rotateeasily about its vertical axis. This arrangement also eliminates theneed for any lubrication.

A half-tone screen 90 of any desired type, such as a Levi screen or anInstant Photo screen, for example, is fixed to the lower end 92 of therotary member 24, as by glue or any other suitable means, so that the(ruled side) lower surface 94 of the screen 90, in its operativeposition, will always be a predetermined distance from the recordingelement 14 on the base -12. In the electrophotographic processvdescribed herein, this distance is about 0.075 inch. The space betweenthe screen 90 and the photoconductive surface 18 will be a function ofthe focal length and magnification of the projection lens. Asillustrated in FIG. 7, the screen 90 comprises a meshwork of lines 96 atright angles to each other, usually rolled on glass. The screen 90 maycomprise between about 33 to 300 lines per inch, depending upon itsapplication. Developed photographic or electrophotographic images thathave been exposed through a screen comprise a plurality of dots ofuniform tone but varying size.

The rotary member 24 is adapted to be rotated about its axis when aforce is applied to the end 72 of the rod 50, and the rod 54 is movedforward so that the pawl 64 engages the ratchet teeth 66 of the rotarymember 24. This force is applied to the rod 54 when the carriage member22 is pushed along the base 12 until the head of the screw 70 engagesthe fixed member 74 and the rod 54 is caused to move, relative to thecarriage member 22, against the spring tension of the spring 62. Thescrew 70 can be extended adjustably to a predetermined length so thatthe rod 54 is forced to move a predetermined distance, against thetension of the spring 62, and the rotary member 24 is caused to rotatethrough a predetermined angle, usually 15 or 30 degrees. The angle ofrotation of the rotary member 24 is also dependent upon the number ofratchet teeth 66. Releasing the force on the rod 54, as by releasing thepush on the carriage member 22 causes the carriage member 22 to moveslightly and the rod 54 to return to its normal springbiased position,as shown in FIG. 5.

The screening unit 10 may be used manually, that is, not in combinationwith a processing head 20, to dispose the half-tone screen 90 apredetermined distance from the recording element 14 at an exposurestation 100, as shown in FIG. 1. Thus, a previously electrostaticallycharged photoconductive surface 18 of the recording element 14 can beexposed by a light image through the half-tone screen 90 to provide ahalf-tone latent electrostatic image on the photoconductive surface 18.The half-tone latent electrostatic image can be developed by anysuitable manual means, or by the processing head 20 in a manner behereinafter described.

The processing head 20 is adapted to cooperate with the screening unit10 to move the latter selectively along a path on the base 12 betweenits park position and its exposure position and to rotate the rotarymember 24 through a predetermined angle between exposures in a processrequiring a plurality of exposures. Referring now particularly to FIGS.3, 4, and 5, the processing head 20 is shown as comprising two separableparts,

fluid applicator means 102 and fluid removal means 104. The fluidremoval means 104 is a member formed with an elongated slot 106extending therethrough from a top surface 108 to a bottom flat surface110 of the means 104. The fluid applicator means 102 comprises arelatively flat rectangular member of insulating material, such asLucite, disposed within the slot 106, well spaced from the side walls ofthe fluid removal means 104 that define the slot 106. A pair ofoutwardly extending tabs 112 and 114 are fixed adjacent the upper edgeof the fluid applicator means 102 and are disposed to rest on the topsurface 108 of the fluid removal means 104, maintaining the fluidapplicator means 102 in position to apply a selected fluid, such as aliquid electroscopic toner, to the recording element 14.

A plurality of flexible tubes 116120 communicate with separate openingsin the fluid applicator means 102, which openings, in turn, communicatewith a metal manifold 122 (FIG. 4) disposed along the lower edge of thefluid applicator means 102. Each of the tubes 116120 is connected to aseparate one of tanks or reservoirs (not shown) through a separate oneof solenoid operated valves 124-128, respectively, as shown in FIG. 1.The manifold 122 is formed with an elongated slit 130 through which aselected fluid can flow onto the photoconductive surface 18 of therecording element 14 when a selected solenoid valve is actuated. Thus,many fluids may be applied sequentially by the processing head 20 in aprocess of developing a latent image on the photoconductive surface 18.

The bottom surface 110 of the fluid removal means 104 is formed with arecess 132 spaced from the walls that define the slot 106. A conduit 134in the fluid removal means 104 communicates with the recess 132 and witha flexible hose 136 which is adapted to be connected to a vacuum pump(not shown) for reducing the gas pressure within the recess 132. Thisreduced pressure removes any of the fluids that have been applied to thesurface 18 of the recording element 14 during the developing process.

Electrostatic charging means is provided in the processing head 20 forapplying an electrostatic charge of a desired polarity onto thephotoconductive surface 18 of the recording element 14. To this end, thebottom surface 110 of the fluid removal means 104 is formed with arecess 138 (FIG. 4) for disposing a thin corona discharge wire 140therein. The Wire 140 is electrically insulated from the fluid removalmeans 104 by a pair of insulators 142 and 144 (FIG. 3), and the wire 140is electrically connected to a unidirectional, high voltage, powersupply 146 (FIG. 1) (about 9 kv.) through a conductor 148. Atappropriate times, a unidirectional voltage of suflicient amplitude isapplied between the wire 140 and the metal base 12 to produce a coronadischarge of a desired polarity around the wire 140 to charge thephotoconductive surface 18 of the recording element 14 uniformly with asuitable electrostatic charge.

The processing head 20 is adapted to be reciprocated over, and slightlyspaced from, the photoconductive surface 18 of the recording element 14so that, in one direction of motion, an electrostatic latent image canbe toned with an appropriate toner, and, in an opposite direction ofmotion, the toned image can be rinsed and the photoconductive surface 18can be electrically charged in preparation for the formation of asubsequent electrostatic latent image. To this end, the processing head20 has two plates 150 and 151 adjustably fixed to the opposite ends ofthe fluid removal means 104. The plate 150 has an ear 152 extendingperpendicularly therefrom. The ear 152 is formed with a hole 154 forreceiving therein a pin 156 on one side of a carriage 158. Similarly,the plate 151 has an ear 160 extending perpendicularly therefrom, andthe ear 160 is formed with a hole 162 for receiving therein a pin (notshown) on the other side of the carriage 158.

The carriage 158 is adapted to move the processing head 20 in areciprocal motion and comprises a plate 164 disposed below the base 12,as shown in FIG. 2. An internally threaded nut 166 is fixed beneath theplate 164 so as to engage a lead screw 167 threadably therein. One endof the lead screw 167 is journaled in a vertical bracket 168 thatsupports one end of the base 12. The other end of the lead screw 167 iscoupled through a suitable gear train 169 to a reversible motor 170. Themotor 170 may be controlled by a reversing switch 172, as shownschematically in FIG. 8, to rotate it in either of two oppositedirections, as desired.

The screening unit 10 may be coupled selectively as by magnetic couplingmeans, to the processing head 20 for movement therewith. Thus, a pair ofelectromagnets 174 and 176 (FIG. 5) are disposed on the upper surface ofthe processing head 20 and fixed thereto by any suitable means. Theelectromagnets 174 and 176 are aligned with the vertical-surfaces 82 and84 of magnetic material of the brackets 76 and 78 on the carriage member22 of the screening unit 10'. Thus, the brackets 76 and 78 are attractedto, and held by, the electromagnets 174 and 176 when the processing head20 is brought into substantial contact with the screening unit 10- andthe electromagnets 174 and 176 are energized. Hence, the screening unit10 can be pushed by the processing head 20 into the park position, thatis away from the exposure station, but the processing head 20 cannotpull the screening unit '10 unless the electromagnets 174 and 176 areenergized.

Alternatively, the electromagnets 174 and 176 can be disposed on thescreening unit 10 and the magnetic brackets 76 and 78 can be disposed onthe processing head 20 to accomplish the aforementioned coupling. Also,while two electromagnets 174 and 176 are shown and described herein, ithas been found that only one electromagnet may be used with satisfactoryresults.

Each of the operations in the electrostatic printing process performedby the apparatus 9 can be sequenced manually in the .process of making ahalf-tone colored composite print from a colored transparency. Thus, forexample, the light projector 16 may be turned on manually by closing aswitch to a suitable circuit (not shown) known in the art. Also, thesolenoid operated valves 124- 128 may be manually energized selectivelyto apply suitable toners sequentially to the processing head 20, and thecorona wire may be energized manually from the power supply 146, whendesired, by closing switches to appropriate circuits, well known in theart.

Referring now to FIG. 8, there is shown a schematic diagram of a portionof a control circuit including means for reciprocating the processinghead 2-0 and for energizing the electromagnets 174 and 176 on theprocessing head 20 selectively. The reversible motor for reciprocatingthe carriage 158 which, in turn, reciprocates the processing head 20, isconnected to a pair of input terminals and 182 of a source ofalternating current through a main double pole-single throw switch 184,a rectifier 186, and the double pole-double throw reversing switch 172.Thus, the processing head 20 can be moved in one direction when the mainswitch 184 is closed and the reversing switch 172 is closed in onedirection. By closing the reversing switch 172 in an opposite direction,the direction of travel of the processing head 20 is reversed.

The electromagnets 174 and 176 are also connected to the input terminals180 and 182 through the main switch 184 and through a switch 188. Thus,by energizing the electromagnets 174 and 176 selectively through theswitch 188, the screening unit 10 can be magnetically coupled to theprocessing head 20 for movement therewith.

A rinse solution solenoid switch and a liquid toner solenoid switch 192,such as for a black liquid toner, for example, is also connected to theinput terminals 180 and 182 through switches 194 and 196, respectively,and the main switch 184. Other circuits, such as circuits for the coronadischarge wire 140, the projector 16 light circuit, and the solenoidoperated valves for the liquid toners and rinse, for example, may alsobe provided through switches 198, 200, etc. in a manner known in theart. While all of the circuits for the apparatus 9 may be actuatedmanually, they may be sequenced automatically by a master controlcircuit (not shown), such as a stepping drum programmer which maycomprise a drum with adjustable plastic cams thereon adapted to actuatea plurality of micro-switches. Such a programmer is Programmer Model No.A31EZ-30, provided by the Tenor Company, New Berlin, Wis.

The screening unit 10 cooperates with the reciprocating processing head20 to make a half-tone colored image on the photoconductive surface 18of the recording element 14 in the following manner: Let it -be assumedthat the screening unit 10 is originally in the park position, as shownin FIG. 5. The processing head 20- is moved over the recording element14 at the exposure station 100 to charge the photoconductive surface 18electrostatically, as by energizing the corona discharge wire 140 whilethe processing head 20 is moved to the extreme right position, shown inFIG. 1. During this movement (to the right), the electromagnets 174 and176 are energized and the screening unit 10 is pulled into the exposureposition, that is, at the exposure station 100. The photoconductivesurface 18 is exposed to a light image through an appropriate filter onthe filter wheel 19 and the screen 90 of the screening unit 10, thusproviding a latent electrostatic image on the previously chargedphotoconductive surface 18. The processing head 20 is now moved to theleft and an appropriate one of the solenoid operated valves 124-128 isactuated to tone the latent electrostatic image with an appropriatetoner.

When the screening unit 10 reaches the end of its path of travel to theleft, the head of the screw 70 on the rod 54 is pushed against thestationary bracket member 74, and the rod 54 is moved against the springbias of the spring 62, causing the rotary member 24 to rotate through apredetermined angle. The length of the screw 70 protruding from the rod54 can be adjusted for the rotation of the rotary member 24 to be about30, or any other desired angle. The screening unit 10 is now ready to bepulled by the electromagnets 174 and 176 when the processing head 20 ismoved again to the right. During this motion, the (first) toned imagecan be rinsed and the photoconductive surface 18 can be charged again inpreparation for the next (second) exposure.

The second exposure of the photoconductive surface 18 can be through anappropriate filter on the filter wheel 19 of the light image projector16, and through the rotated screen half-tone 90, producing a secondlatent electrostatic image on the photoconductive surface 18. The secondlatent electrostatic image is toned by moving the processing head 20 tothe left again and actuating an appropriate one of the solenoid valves124-428 to allow a desired color toner to develop the latent image.During this motion, the screening unit 10 is once more pushed to itsextreme left position, thereby causing a force to be applied to the headof the screw 70 by the bracket 74 and, causing the screen 90 to turnthrough an angle of 30 again.

If it is desired that the next (third) exposure, through another filteron the filter wheel of the light image projector 16 be made withoutusing the half-tone screen 90, the electromagnets 174 and 176 can bedeenergized, as by opening the switch 188, and the screening unit 10 isleft in the park position when the processing head 20 is moved to theright. If, however, it is desired that the third exposure be madethrough the screening unit 10, the electromagncts 174 and 176 areenergized and the screening unit 10 is pulled along with the processinghead 20 as the latter is moved to the extreme right. As many exposuresas desired may thus be made on the photoconductive surface 18 at theexposure station 100, either through the half-tone screen of thescreening unit 10 or without the half-tone screen 90, demanded by anyparticular developing process.

We claim:

1. A screening unit for use with a recording element,

said screening unit comprising:

a rotary member adapted to rotate about an axis,

a screen,

means supporting said screen on said rotary member substantiallyperpendicularly to said axis,

a carriage member adapted to be positioned over said recording element,and

means on said members for supporting said rotary member on said carriagemember for rotation about said axis with said screen at a predetermineddistance from said recording element when said carriage member ispositioned thereover,

said carriage member comprising wheel means, whereby it can be movedalong a path over said recording element,

said carriage means having a through opening, and

said means on said members for supporting said rotary member on saidcarriage member comprising a first flange adjacent the periphery of saidrotary member, and a second flange defining a portion of said throughopening, whereby said rotary member is disposed for rotation about itsaxis within said through opening with said first and second flanges in asubstantial abutting relationship.

2. A screening unit for use with a recording element,

said screening unit comprising:

a rotary member adapted to rotate about an axis,

a screen,

means supporting said screen on said rotary member substantiallyperpendicularly to said axis,

a carriage member adapted to be positioned over said recording element,and

means on said members for supporting said rotary member on said carriagemember for rotation about said axis with said screen at a predetermineddistance from said recording element when said carriage member ispositioned thereover,

said rotary member comprising rotating means to rotate it about itsaxis, and

spring-biased reciprocating means being mounted on said carriage memberand being cooperatively associated with said rotating means on saidrotary member to rotate said rotary member when a force is applied tosaid reciprocating means against the spring bias thereof.

3. A screening unit as described in claim 2, wherein said rotating meanscomprise ratchet teeth formed in a circular pattern, and

said spring-biased reciprocating means comprises a rod and a pawlmounted adjacent an end of said rod and adapted to engage said ratchetteeth, whereby reciprocating said rod in one direction causes saidrotary member to rotate about said axis.

4. A screening unit for use with a recording element,

said screening unit comprising:

a carriage member,

means cooperating with said carriage member to support it over saidrecording element,

a through opening in said carriage member extending in a directiontransversely to said recording element when said carriage member is oversaid recording element,

a rotary member having an axis of rotation and a pair of opposite ends,

a screen,

means to support said screen adjacent one of said ends substantiallyperpendicularly to said axis of rotation,

means to maintain said rotary member for rotation about said axis andwithin said through opening in said carriage member with said screen ata predetermined distance from said recording element when said carriagemember is supported thereover, and

means on said rotary member for rotating it about said axis.

5. A screening unit as described in claim 4 wherein said recordingelement is supported on a base, and

said means on said carriage member to support it over said recordingelement comprises a plurality of roller means fixed to said carriagemember and adapted to engage said base, whereby said carriage member maybe rolled over said recording element.

6. A screening unit as described in claim 5 wherein guide means arefixed to said carriage member for cooperating with said base to guidesaid carriage member along a predetermined path when said carriagemember is moved over said recording element.

7. A screening unit as described in claim 4 wherein said carriage membercomprises a plate having upper and lower major surfaces,

said through opening extends between said major surfaces, and

said means to maintain said rotary member for rotation about, its axisand within said through opening comprise an inwardly extending firstflange on said carriage member which defines and narrows a portion ofsaid through opening adjacent said lower major surface, an outwardlyextending second flange on said rotary? member adjacent the other endthereof, and friction reducing means disposed between said first andsecond flanges whereby said rotary member may be rotated easily aboutsaid axis.

8. A screening unit as described in claim 4 wherein said means on saidrotary member for rotating it about its axis comprises a plurality ofratchet teeth formed adjacent the other end of said rotary member.

9. A screening unit as described in claim 8 wherein a rod is mounted onsaid carriage member for reciprocal movement with respect thereto,

a pawl is coupled to one end of said rod and adapted to engage saidratchet teeth for rotating said rotary member when said rod is moved inone direction of its reciprocal movement,

spring means biases said rod in an opposite direction to said onedirection, and

the other end of said rod comprises means to engage a force for movingsaid rod against said spring means,

whereby said pawl can engage said ratchet teeth and rotate said rotarymember. 10. In apparatus of the type for processing a recording elementto reproduce an image thereon, wherein means are provided (1) to exposesaid recording element with a light image of said image to be reproducedto form a latent image, and (2) to reciprocate a processing head acrosssaid recording element to develop said latent image, the improvementcomprising:

a screening unit comprising a screen adapted to be disposed over apredetermined location and spaced a predetermined distance from saidrecording element when said recording element is exposed by said lightimage, whereby said recording element is exposed through said screen,and

means including said processing head to move said screening unitselectively into, and away from, said predetermined location during theprocessing of said recording element by said apparatus.

11. In apparatus of the type described in claim 10',

wherein said screening unit comprises a carriage member and a rotarymember adapted to be carried by said carriage member,

said screen is attached to said rotary member substantiallyperpendicularly to the axis of rotation of said rotary member, and

said means to move said screening unit selectively includes magneticmeans cooperatively associated with said processing head and saidscreening unit for coupling said processing head to said screening unit.

12. In apparatus of the type described in claim 10,

wherein said apparatus comprises, in addition,

a base for supporting said recording element,

said screening unit comprises, in addition,

a carriage member, wheel means on said carriage member adapted to engagesaid base for rolling thereover, and guide means for guiding saidcarriage member over said base along a predetermined path including saidpredetermined location,

controllable electromagnetic means fixed to said processing head, and

magnetic means fixed to said carriage member and adapted to be attractedto said electromagnetic means when said processing head is adjacent saidcarriage member, whereby said carriage member and said processing headcan be moved together simultaneously when said electromagnetic meansareenergized.

References Cited UNITED STATES PATENTS 1,226,838 5/1917 Wolber -36 JOHNM. HORAN, Primary Examiner US. Cl. X.R.

